Conversions of hydrocarbons or substituted hydrocarbons by catalytic reactions such as oxidation are significant commercial processes. A direct catalytic partial oxidation of a hydrocarbon feedstock, such as an alkane or alkene, with high activity and selectivity would be an important process capable of commercial use. The current example of a commercial direct oxidation of an alkane is oxidation of butane to maleic anhydride using a vanadium phosphorus oxide (VPO) catalyst system. There is an ongoing need for new and improved processes to produce acrylic acid and maleic acid by partial oxidation of inexpensive feedstocks such as propane and butane.
A desirable catalyst system and process would operate at relative low temperatures and pressures with high catalyst productivity and selectivity.
Heteropolyacids have been widely described as oxidation catalysts, such as to oxidize alkanes to unsaturated carboxylic acids. Further, heteropolyacids substituted with a wide variety of elements have been described generally as catalysts for oxidation and other purposes. Polyoxometallates and heteropolyacids are described in Pope et al., Heteropoly and Isopoly Oxometallates: Inorganic Chemistry Concepts, Springer-Verlag, New York (1983) (ISBN: 0387118896), incorporated herein by reference. Pope et al. and others have described numerous uses of heteropolyacids (“HPA's”) and polyoxometallates (“POM's”) in catalysis such as oxidation of propylene and isobutylene to acrylic and methacrylic acids, oxidation of aromatic hydrocarbons; olefin polymerization; ammoxidation; oxidation of crotonaldehyde or butadiene to furan; dehydration of alcohols; oxidative coupling of alkyl benzenes or heterocycles; epoxidation; and hydrodesulfurization.
Ueda and co-workers (“Partial Oxidation of Propane to Acrylic Acid over Reduced Heteropolymolybdate Catalysts,” Chemistry Letters 1995, pp. 541–542 (1995); “Catalytic performance for propane selective oxidation and surface properties of 12-molybdophosphoric acid treated with pyridine,” Applied Catalysis A: General, vol. 182, pp. 357–363 (1999)) have described partial oxidation of propane to acrylic acid using a heteropolymolybdophosphoric acid treated with pyridine and heated in the catalyst preparation. However, the Ueda et al. catalyst and process have been found to have relatively low activity with limited catalyst lifetimes.
Examples of heteropolyacids and polyoxometallates incorporating a wide variety of elements and other constituents and used in catalyst systems include U.S. Pat. Nos. 5,990,348, 6,060,419, and 6,060,422, 6,169,202. Examples of other mixed metal oxide catalyst systems include U.S. Pat. Nos. 5,380,933 and 6,294,685.
The abundance and low cost of light alkanes has motivated the search for new catalytic materials that can accomplish selective oxidation processes. The conversion of n-butane to maleic anhydride over V—P—O catalysts with molecular oxygen is commercially well established. Other reactions of current interest are the production of acetic acid from ethane and acrylic acid from propane. Polyoxometallates are among the numerous catalytic materials that have been investigated for each of the aforementioned reactions. Typically, different polyoxometallate compositions have been used for each alkane. These compounds (and other mixed metal oxides) have not been found to perform as well as V—P—O catalysts for the conversion of n-butane to maleic anhydride, or as well as mixed metal oxides containing Mo—V—Nb—Te or Mo—V—Nb—Sb for conversion of propane to acrylic acid.
We have discovered a new catalyst system that achieves selective oxidation of both n-butane and propane. Li et al. (Appl. Catal. A., vol. 182, pp. 357–363 (1999)) have reported that a solid obtained by treating molybdophosphoric acid, H3PMo12O40 (denoted as PMo12) with pyridine followed by activation in nitrogen at 420° C. exhibits catalytic activity for oxidation of propane to acrylic acid. Ueda et al. also showed that molybdovanadophosphoric acid (PMo11V) similarly treated gives a less active and selective catalyst. Our preferable catalysts are obtained from PMo12 and PMo11V, exchanged sequentially with niobium oxalate (giving NbPMo12 and NbPMo11V) and pyridine (giving NbPMo12pyr and NbPMo11Vpyr) in aqueous media, followed by heating in flowing non-oxidizing atmosphere.
Catalysts of this invention exhibit substantially higher productivities (in terms of space time yield (STY)) and at least comparable selectivities, as well as the ability to operate efficiently at lower temperatures (˜300° C.). In addition to hydrocarbon-rich conditions, these new catalysts perform well in more typically-studied hydrocarbon-lean environments (e.g., C4/O2=1/10).
These new catalysts are effective for more than one feedstock. Propane also may be oxidized selectively to a variety of partially oxidized products. Interestingly, in addition to forming acrylic and acetic acids, respectively, the catalyst produces substantial amounts of maleic acid.
Although catalytic uses of heteropolyacids have been described widely, there is a need for catalyst materials that are capable of partial oxidation of hydrocarbons and other compounds under relatively mild conditions at high activity. Applicants have discovered an effective catalyst material based on a suitable Group 3–15 or lanthanide metal salt of a polyoxoanion that has been activated by partial reduction and phase transformation. This catalyst material is capable of oxidizing an alkane such as propane or butane to acrylic or maleic acids with high activity and selectivity.